While increasing evidence suggests that maternal exposure to high levels of ambient air pollutants during pregnancy plays a critical role in producing adverse pregnancy outcomes, most of the early studies focused on birth outcomes (such as birth weight and preterm birth), very few studies, however, have investigated air pollution in relation to fetal growth measured through ultrasound, a sensitive index and novel approach to examine how early air pollution effects alter fetal growth as the measurement at birth is considered a delayed measure of fetal growth. Furthermore, early studies have been largely conducted in western populations, with few studies conducted in China which has the world's most polluted cities and the largest world burden in adverse pregnancy outcomes. The severe ambient air pollution in China is unfortunate but provides a unique environment to study ambient air pollution and fetal growth. Building upon an on-going large prospective birth cohort study (n=6,000) in 2 Chinese cities which have different types and levels of air pollution, we propose a study to examine the hypothesis that heavy maternal exposure to ambient air pollutants (SO2, NO2, PM10, CO, and O3) during pregnancy results in reductions in fetal growth, and the risk of adverse fetal growth varies by windows of exposure during pregnancy. We will take a novel approach of collecting multiple ultrasound measurements of fetal growth during pregnancy through the on-going prospective birth cohort study. The outcomes of interest of the proposed study are 18,000 ultrasound measures (three measurements for each subject) of biparietal diameter (BPD), occipital frontal diameter (OFD), head circumference (HC), femur length (FL), and abdominal circumference (AC) at 13th, 26th, and 35th week gestation. The daily levels of ambient air pollutants (SO2, NO2, PM10, CO, and O3) will be collected from each cities' EPA stations and major potential confounders will be collected through in-person interviews. Linear mixed effects models will be used to estimate associations between ambient pollutant levels and fetal growth over pregnancy. This study will be the largest among very few prospective studies to look at the impact of air pollution on fetal growth controlling for major confounding factors. In addition it is the only air pollution study in which multiple ultrasounds will be conducted as an objective measure of growth during various stages of gestation. Because China has the world's most polluted cities and the largest burden of adverse pregnancy outcomes, these findings will have important implications for regions of the world with similar and lower levels of pollution, where it would be difficult to investigate the association meaningfully.